The invention relates to a method for operating a superconducting device, comprising a coated conductor with a substrate and a quenchable superconducting film, wherein the coated conductor has a width W and a length L.
Such a superconducting device is known for example from EP 2 192 629 A1.
Superconducting devices are used in different ways, in particular to transport electric currents, for current conditioning such as in fault current limiters, or for generating high strength magnetic fields.
Superconducting devices comprise a conductor which may, at a temperature below the so called critical temperature Tc, carry an electric current at practically no ohmic losses. In order to achieve said low temperature, the conductor is typically cooled with liquefied gases such as liquid helium. Further, to have a superconducting state, it is also necessary to stay below a critical current density and below a critical magnetic field with the conductor.
When using high temperature superconductor (HTS) materials, e.g. yttrium barium copper oxide (YBCO) material, higher temperatures, current densities and magnetic fields become accessible. HTS material is typically used as a film (or coating) on a normally-conducting or insulating substrate.
A difficulty when employing superconducting devices is the risk of a sudden loss of the superconducting state, also called a quench. If a region of a superconducting film quenches, a high electric current has to pass through the region now normally conducting, what causes a considerable heating of said region. The high current and heating may damage the superconducting material, what is also called a burnout.
Generally, it is desired that a superconducting device can survive a quench, so it can be used again e.g. after recooling of the device. In order to avoid a burnout of the superconducting material, it is known to protect superconducting films with shunt resistance.
EP 2 117 056 B1 discloses an elongated coated conductor, comprising a superconducting film on a substrate, covered by a first metallic member, and electrically connected to a resistive member running in parallel to the elongated conductor via regularly spaced bridge contacts. The resistive member is spaced apart from the elongated conductor, so the resistive member is thermally decoupled from the elongated coated conductor. The resistive member provides an external shunt protection.
In case of a quench in the superconducting film, the major part of the electric current is rerouted around the quenched region through the resistive member, so the current strength through the elongated conductor in the quenched region is reduced. The major heating occurs in the resistive member then, and not in the elongated conductor.
It is also known to cover or encapsulate superconducting films with a normally conducting stabilization layer, e.g. made of copper, compare U.S. Pat. No. 7,774,035 B2.Again, in case of a quench, the electric current is to be rerouted through the stabilization layer in order to preserve the superconducting films. The stabilization layer provides an internal shunt system for a coated conductor.
Coated conductors with external shunt protection are difficult to manufacture. Internal shunt protection may reroute a major part of the electric current away from the superconducting film, but heating may still damage the superconducting film. Therefore, even when using known external and internal shunt protection, a burnout of the superconductor film may still occur.
It is the object of the invention to further reduce the risk of a burnout of a superconducting device in case of a quench in its superconducting film.